<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>三角形</title>
    <style>
        #div{
            width: 100px;
           height: 100px;        }
        span{
            display: inline-block;
            border: 100px solid ;
            border-color: red transparent transparent transparent;
            transform: rotate(180deg);
        }
    </style>
</head>
<body>
<div id="div">
    <span></span>
</div>
</body>
</html>